The invention relates to a propulsion control system for motor vehicles to prevent undesirable spinning of the wheels especially in a multi-axle drive vehicle.
This type of system is shown in U.S. Pat. No. 4,484,280. That text concerns a motor vehicle with two axles, one of which, the rear axle or its wheels, are driven. Therefore the propulsion control system affects the rear wheels. For calculating the slip of the driven wheels or of the wheels that are to be controlled, these types of propulsion or wheel slip controls require the wheel circumference speeds of these wheels and the speed of the vehicle. Usually a speed comparison is carried out for each side of the vehicle, in which case the wheel that is not driven supplies the required vehicle speed information.
In the case of vehicles with all-wheel drive, the front wheels cannot be used for forming a driving speed signal, because, in the case of an excess of engine torque, all wheels may have higher slip values.
Systems are known that determine the driving speed by means of visual devices or by means of the Doppler effect. However, these systems are very costly.
It is the objective of the invention to provide the propulsion control system that is known for vehicles with one driven axle operable also for vehicles with all-wheel drive without requiring costly systems for determining the driving speed.
This and other objects of the invention are obtained by determining the reference speed required for the slip threshold information by integrating the speed of an uncontrolled drive wheel by an integrating circuit with a time constant. The time constant is derived by integrating the longitudinal acceleration of the vehicle, subtracting the integrated signal from the longitudinal acceleration and adding a constant value assigned the constant value of acceleration to the difference. This signal is used to vary the time constant of an integrating circuit which is between a signal of one of the wheels sensors assigned as the vehicle speed and a multiplier which multiplies the integrator's output signal with a factor to produce the threshold values. A circuit is provided for maintaining the output signal of the integrator in the acceleration circuit constant at the momentary value that it had at the start of the brake actuating signal during the duration of the brake actuation signal period. The integrating circuit for the slip threshold circuit includes circuitry for preventing the output signal of the integrating circuit of becoming larger than its input.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.